1. Field of the Invention
The present invention relates to a method for pulling single crystals from a melt of semiconductor material, in which a monocrystalline seed crystal grows to form a single crystal, the seed crystal being dipped into the melt and raised in a controlled manner in the vertical direction with respect to the melt, while the melt forms a molten pool which is held on a support body only by the surface tension and by electromagnetic forces due to an induction coil. The invention furthermore relates to an apparatus for carrying out the method.
2. The Prior Art
The growth of single crystals by such a method, also referred to as "pedestal pulling," differs from the method of growing single crystals most frequently used in semiconductor technology, the so-called Czochralski method, mainly in that, in the case of the latter, a crucible accommodates the melt of the semiconductor material. As a rule, the internal walls of the crucible which come into contact with the melt in this process are made of quartz, which slowly begins to dissolve in the melt at high temperatures. As a result, impurities such as, for example, oxygen, boron and aluminum originating from the quartz are incorporated into the lattice of the growing single crystal. In some applications in semiconductor technology, for example in the production of electronic power components, however, single crystals which are not contaminated with oxygen, or are contaminated only with small amounts of it, are required as raw materials. A crucible-free crystal pulling technique, such as the "floating zone" or the "pedestal pulling" technique is preferred for the production of such single crystals. In the case of the method first mentioned, a cylindrical ingot of polycrystalline semiconductor material is melted at one end by means of an induction coil, a monocrystalline seed crystal is dipped into the melt and a relative movement of ingot and coil ensures that a molten zone slowly migrates through the polycrystalline ingot from the seed-crystal end. The semiconductor material melted in the molten zone recrystallizes in monocrystalline form, with the result that a single crystal is finally obtained. A disadvantage of this method is that the mass of the polycrystalline ingot limits the mass of the single crystal and the polycrystalline ingots used cannot have any desired length and any desired diameter for equipment reasons. Since it has been found advantageous for the diameter of the polycrystalline ingot and of the monocrystalline ingot to be in an approximate ratio of 1:1, polycrystalline ingots having large diameters are also required for the production of large single crystals. It can be observed that parts may chip off from the edges of large polycrystalline ingots which have diameters of 150 mm and greater and which are normally produced by depositing the semiconductor material on a so-called slim rod, because of the brittleness of the semiconductor material even under very low mechanical, or thermal, stress. If this takes place during the growth of a single crystal, there is a danger of the formation of dislocations in the crystal lattice, with the result that the crystal becomes unusable for electronic applications.
U.S. Pat. No. 2,961,305 describes a method of producing single crystals by "pedestal pulling". In the method disclosed therein, a disk of semiconductor material resting on a segmented support body is first melted inductively. Then a monocrystalline seed crystal is dipped into the melt and raised in a controlled manner in the vertical direction with respect to the melt. As the molten semiconductor material is used up, the seed crystal grows to form an ingot-shaped single crystal. Since the segmented support body is also composed of the semiconductor material in this prior art method, and since the support body is directionally melted as a consequence of the heat transmission from the melt itself, which rests on it, and on the moving inductive heating system, a single crystal can be grown whose maximum achievable mass is approximately equal to the sum of the masses of the disk and of the support body. In addition to limiting the achievable size of the single crystal in an unfavorable manner in this prior art method as well, a particular disadvantage in this case is that the support body produced with high costs is destroyed in the course of the method.
U.S. Pat. No. 3,936,346 discloses another example of the "pedestal pulling" method. The method described therein is that a polycrystalline semiconductor ingot is continuously pushed from below into a cage formed by induction coils and open at the bottom and top. The part of the semiconductor ingot which is precisely in the cage melts and forms a melt which does not touch the sidewalls of the cage because of the surface tension of the semiconductor material and because of the electromagnetic forces generated by the coils. At the beginning of this method, a melt is generated in the cage by introducing the polycrystalline ingot. Then a monocrystalline seed crystal is dipped into the melt and raised in a controlled manner in the vertical direction with respect to the melt, and material continuously grows on the seed crystal from the melt. Feeding the polycrystalline semiconductor ingot forward ensures that the amount of melt in the cage remains constant. In this prior art method, too, there is the disadvantageous dependence, already mentioned, of the mass of the single crystal on that of the polycrystalline sacrificial ingot.